It might just be "because they needed to make an interesting game" but why didn't the Lutece twins or one of them just go back and do what the Anna/Elizabeths did and drown Booker? I don't get why they needed him to do all that pointless shit when they had the tech to open those tears.

My assumption is that Lutece device is limited in where it can create points of entry into other realities. Perhaps only as far as back as the creation of the device, and forward indefinitely for all realities created from that point on. It's only Elizabeth who can travel far enough back to the point of the Comstock/Booker split, and put the Booker she brings with her into the Booker/Comstock body (so it's not two Bookers at the baptism, but the one that needs to die).

I don't think they wanted Booker dead. Robert wanted to give Booker a Chance to reclaim his daughter, because he was the reason Comstock stole her in the first place. They weren't trying to stop Comstock, they were attempting to reunited a father and his daughter. Elizabeth and Booker are the ones that decide to 'stop' Comstock, which leads to Booker's death. At least, that was my interpretation.

you are, in fact, mistaken. One of Rosalind Lutece's Voxophones in the game discusses the motivation behind the Booker and Elizabeth plan.

After Rosalind Lutece found Robert in Booker's dimension, she wanted them to be together. In all of her voxophones she simply refers to him as "brother," and he is one of her most discussed topics. HOWEVER, some time after helping Comstock retrieve Anna from Booker's dimension, Robert was filled with regret and sought to undo his actions.

As a result, he told Rosalind (quoted from Rosalind's voxophone) "we must return the girl from where she came or part ways." The voxophone in question ends with Rosalind saying "So I will play my part, though I know that it will only end it tears." [tears and in crying].

As mentioned by BoyWonder, Robert wanted to bring Anna back to Booker. However, the Lutece's were unable to intervene directly due to the fact that Comstock had them killed by Fink (yes, they are dead in Comstock's dimension. Someone in /r/Bioshock was able to explain their ability to exist across the multiverse following their deaths, but I do not remember it at the moment).

Therefore, Robert devised the plan referred to by Rosalind as "The Thought Experiment" in the opening scene of the game: retrieve Booker DeWitt from his dimension and bring him through a tear into Comstock's dimension. Booker would then have his opportunity to reunite himself with Anna, which was what Robert wanted.

However, Rosalind was able to see further down the line, and anticipated the events at the end of the game. Specifically, Rosalind understood that her and Robert's actions would create "a loop," an anomaly in time and space which, due to its nature, would be forced to occur over and over again for all of time (the game's title of 'Infinite' begins to make more sense).

Furthermore, Rosalind understood that the only way to close the loop would be for the final scene of the game to take place: for Booker DeWitt to be returned to the moment before his timeline branched at the river of baptism, and for him to die there before the choice was made.

What was unknown to Rosalind was that closing Booker's loop would replace the Booker/Comstock branch in his timeline with a single Booker branch. This new branch is seen in the incredibly short clip after the credits, where Booker is back in his office with Anna still in his possession. Unfortunately for the Lutece's, however, this new reality is one in which they will never meet. And, in fact, Rosalind likely never existed due to the Comstock reality never existing.

Wow, that was a lot longer than I expected, but hopefully that helps explain the Lutece's motivations. It's amazing what you can learn from a small hand full of Voxophone!.

The loop they create is that Elizabeth drowning Booker prevents the events of the game from ever taking place. However, for Elizabeth to drown Booker, the events of the game HAD to take place. So events of the game, and the drowning, occur infinitely in their own time loop.

However, the Lutece's were unable to intervene directly due to the fact that Comstock had them killed by Fink (yes, they are dead in Comstock's dimension.

They were not killed. According to the Voxophone left by Rosalind Lutece after they were "killed": They are not dead and her theory is that they "are scattered amongst the possibility space."

Being scattered left them "unstuck" if you will, unable to use direct action. They could still influence Booker through indirect action. Although that falls apart right at the beginning when they are rowing Booker to the lighthouse... seems pretty direct. Maybe they are replacing other "variables" in that reality in a temporary fashion. There was someone already rowing to the lighthouse and the Lutece's replaced them with themselves and Booker.

The mission is never to kill Comstock, it's to get Elizabeth. The 122 before him are implied to be just the failed attempted, which means that could just be the Bookers that died or couldn't rescue Elizabeth. They never push Booker to kill Comstock. Again, both of our theories are assumptions, because it is never completely revealed in the game.

I don't think so. Rosalind seems pretty content with the situation she's in, and it's mentioned in several voxaphones that Robert is "trying to set things right" which is implied to mean with Annabeth and Booker.

As I mentioned in the giant post above, Comstock wasn't just "going to" kill the Lutece's, he succeeeded. One of the voxophones in the Market district outside Comstock house discusses someone who was visited by the Lutece twins several days after they were confirmed dead.

I thought about that too and decided since they were trapped between realities, because Fink sabotaged their machine, they could not go back and change their own choices, whereas Elizabeth could when she possessed her full powers at the end. Perhaps the Lutece twins did go back and stop Commstock in particular realities. We only got to see one out of infinite realities.

Those infinite realities all start at the same place, when Booker was baptized and Comstock was born. If Comstock was never born, Booker would never sell Elizabeth, and Columbia would never come to exist. At least, that's my take on it.

The drowning at the end of the game occurs before the baptism. If you think of Booker's timeline like a tree, then the baptism represents a point in the trunk of the tree where it splits in half. One half of the tree (Branch A) represents all possible timelines where Booker refuses the baptism and remains booker. The other half (Branch B) are all the possible timelines where he becomes Comstock.

So by downing Booker BEFORE he ever reaches the baptism circle (before the moment of the branch) he is NEVER capable of making that choice. Every possible Comstock (and every possible associated Booker) is erased as a result of the downing occurring before the baptism. Also, as a result, every possible Anna/Elizabeth is erased because Booker was never alive to marry Annabelle and have Anna.

This is not true. When Booker is drowned in the end, it creates a time paradox. Consider the following: if Booker dies, then there will be no Comstock, and without Comstock he wouldn't have stolen Anna from Booker, and Liz would have never been raised by Comstock in the tower, and without Liz, no one will drown Booker at the baptism.

The above scenario is a paradox, and in the Bioshock Infinite universe, they assume the Novikov's self-consistency theory (pointed out by a fellow redditor, forget the name), which states that any event creating a paradox = 0, meaning that it is impossible for the paradox to happen.

Thus the universe eliminates all the "branches" where Booker goes through with the baptism and becomes Comstock, leaving only the "branches" where Booker doesn't go through with the baptism. This is why all the Elizabeths disappear in the end, and also why the ending scene shows Booker WITH Anna.

but the booker we play as has already experienced the baptism in his lifetime, wouldn't have made more sense to kill one that never went to columbia yet, guess it wouldnt matter in the end. and by the way, what is the point of the scene after the credits again? is the game starting over? that the cycle is repeating?

I don't get how all of those Elizabeth's appeared and all vanished with that either? No idea why killing the origin in one area changes anything, they didn't go back far enough to guarantee that would never happen.

though there may be other explanations as well, the twins are established as characters who love to experiment. They viewed the entire thing as a thought experiment. Combine that with them having all the time in the world and you have the path they chose to take